The present invention relates to control systems for adjusting the color-to-color registration of multicolor web-fed printing press systems.
In multicolor web-fed printing press systems, a web of material (e.g., paper) is sequentially driven through a series of print cylinders, each using ink of a different color, which cooperate to imprint a multicolor image on the web. To provide an accurate and clear multicolor image, the rotational and lateral position of each print cylinder must be precisely aligned, i.e., proper registration of the respective colors must be maintained.
Historically, the registration of the various print cylinders in multicolor systems was maintained manually. A pressman would examine signatures (printed images) at the output of the press, and manually enter estimated lateral and rotational offset values into an electromechanical register control system to effect necessary corrections. Retention of color registration in such systems requires the constant attention of the pressman, since registration is often lost due to a number of uncontrollable variables in the web material and press hardware.
Automatic registration control systems for multi-color web-fed printing press systems are, in general, known. An example of a closed-loop register control system is the commercially available Quad/Tech RGSIII register control system, a product of the assignee of the present invention. The RGSIII system utilizes a single photosensor cooperating with register marks printed on the web by the individual cylinders to provide position feedback information indicative of the registration of the respective print cylinders relative to a designated reference print cylinder. More particularly, each print cylinder produces a specific register mark forming part of a register pattern. The photosensor provides a feedback signal indicative of the register pattern, which is analyzed to determine the lateral and rotational registration of the respective print cylinders vis-a-vis the reference cylinder. Registration error signals are produced for the respective print cylinders and are employed to effect position correction.
Each register mark in the RGSIII system comprises two triangular sections disposed, if properly aligned, to produce two voltage pulses of equal time duration. The photosensor is positioned to focus at the center of the register pattern. As paper motion takes the pattern past the scanner, a series of voltage pulses are produced. To track any lateral movement of the web, the duration of the respective pulses generated by the marks is monitored; if the focal point of the scanner is not centered, the two pulses produced by the respective triangular sections of the mark will not be of equal duration. When such a condition is sensed, the system generates control signals to reposition the printing cylinder to center the pattern. If the printing cylinder associated with a mark is rotationally out of registration, the register mark produced will not be properly aligned in the register pattern, and the two voltage pulses produced by the register mark will be advanced or delayed relative to the pulses produced by the regigster marks associated with the other cylinders. Deviations from predetermined values of the position of the center points of the register marks relative to a pattern reference point are detected and control signals generated accordingly.
Previous automatic registration control systems, which utilize only a single photosensor scanner, scan a single straight line as the paper travels under the focal point of the sensor. This requires the register marks to be sufficiently wide to be sensed by the scanner in worst-register situations, often at least 0.2 inch total in width. Moreover, web "weave" (spurious lateral movement of the web) tends to require that a still wider registration mark be employed. This is disadvantageous; it is desirable that the registration mark be as small and unobtrusive as possible.
Prior art registration control systems using a single triangle registration mark are additionally disadvantageous in that variations in the density of the inks can often be erroneously perceived by such systems as registration errors.